Hi all, quick question. I purchased a 5 gallon carboy and a #7 stopper with hole from morebeer.com. I racked my Rasberry Wheat into in last week, and I pushed the stopper in and secured the airlock (after the stopper was in). The next morning I found the stopper almost all the way out of the hole. When I touched it it felt like it would just fall out. So I jamed the thing in as far as I could, but yet again it tried to work its way out.

I always had a heck of a time with those, especially when they were still wet. All I could ever think to do was to dry it with a paper towel--I know, technically it's not sanitized then. Then I switched to those snap-on flexible carboy caps (I forget what they're called).

This is just a guess, but if the beer wasn't fully fermented when you transferred it to the secondary it would still produce CO2. This will increase the pressure in the headspace above the beer/wort. As this pressure increases it will exert an increasing force on the sides of the vessel, top of the vessel and the surface of the liquid. Whichever of the three is the 'weakest link' will determine what will happen.

For example, when you bottle beer the cap is snug, the bottle isn't cracked, therefore as the pressure in the headspace of the bottle increases the excess CO2 is dissolved in the beer thus producing carbonation.

Suppose now that you don't have a snug cap. What would happen to the cap? After some amount of pressure is built up in the bottle the cap would probably fly off.

I think that is what's happening here. The wort probably wasn't fully fermented when transferred, and the stopper doesn't have a good enough hold on the carboy to prevent it from slipping out because of the increasing pressure.

This explanation also explains why after a certain amount of time the stopper fits snugly in the hole on top of the carboy.